It’s mostly the velocity. Orbiting a planet just means having enough kinetic energy to fall toward a planet, but continually overshoot and miss. They need some way to slow down dramatically, which is usually accomplished with rocket thrusters. If they just strapped on parachutes and jumped out, they’d orbit the Earth alongside the ISS.

ISS is around 400km of altitude. There is still a little bit of atmospheric friction there so you’ll definitely end up on earth but it will take years and you’ll probably burn while passing the Kármán line at 100km.

The good thing is that you’ll feel nothing at that moment cause you’ll be dead for a while due to no oxygen reserves.

The only way to escape is what they do today; a re entry pod with thrusters to do an entry burn at a scheduled time (if you burn at the wrong time and place you can go “higher” towards open space instead of back to earth but you also want to aim at your landing place and not end up in the middle of manhattan or a minefield), a heat shield for passing the atmosphere and then parachutes to soften the landing.

If you are curious about rocket science and like games, I suggest you to buy Kerbal Space Program 1, there’s tons of mods and you’ll learn while having fun.